Synchronizing nodes in an optical communications system utilizing frequency division multiplexing

Abstract

Attenuation caused by dispersion in an optical fiber communications system is compensated. A number of low-speed channels is to be transmitted across an optical fiber. Each low-speed channel is allocated a different frequency band for transmission. The attenuation caused by dispersion is estimated for each of the frequency bands. The power of each low-speed channel is adjusted to compensate for the estimated attenuation. The power-adjusted low-speed channels are frequency division multiplexed together to produce an electrical high-speed channel suitable for transmission across the communications system.

1. In an optical fiber communications system including a transmitter node coupled to a receiver node by an optical fiber, a method for synchronizing the receiver node with the transmitter node, the method comprising:
 at the transmitter node:
 generating a reference signal; 
 synchronizing the transmitter node with the reference signal; 
 wherein said synchronizing the transmitter node comprises adjusting the data rate of a data signal to match the rate of the reference signal; 
 modulating the reference signal onto an optical signal in a first frequency band and modulating the data signal onto the optical signal in a second frequency band, the first frequency band being different from the second frequency band; and 
 transmitting the optical signal across the optical fiber to the receiver node; and 
 
 at the receiver node:
 receiving the optical signal; 
 recovering the reference signal and the data signal from the optical signal; and 
 synchronizing the receiver node with the recovered reference signal; 
 wherein said synchronizing the receiver node comprises adjusting the data rate of the recovered data signal to match the rate of the recovered reference signal. 
 
 
   
   
     2. The method of  claim 1  wherein:
 each of the transmitter node and the receiver node includes a local oscillator; 
 synchronizing the transmitter node with the reference signal comprises synchronizing a local oscillator at the transmitter node with the reference signal; and 
 synchronizing the receiver node with the recovered reference signal comprises synchronizing a local oscillator at the receiver node with the recovered reference signal. 
 
   
   
     3. The method of  claim 1  wherein:
 modulating the reference signal and the data signal onto an optical signal comprises:
 generating a harmonic of the reference signal; and 
 modulating the harmonic onto the optical signal; and 
 
 recovering the reference signal and the data signal from the optical signal comprises:
 recovering the harmonic from the optical signal; and 
 frequency dividing the harmonic to recover the reference signal. 
 
 
   
   
     4. The method of  claim 1  wherein:
 modulating the reference signal and the data signal onto an optical signal comprises:
 frequency division multiplexing the reference signal with a plurality of electrical low-speed channels to form an electrical high-speed channel; and 
 converting the electrical high-speed channel from electrical to optical form to form the optical signal; and 
 
 recovering the reference signal and the data signal from the optical signal comprises:
 converting the optical signal from optical to electrical form to recover the electrical high-speed channel; and 
 frequency division demultiplexing the reference signal from the electrical high-speed channel. 
 
 
   
   
     5. The method of  claim 4  wherein, in the electrical high-speed channel, the reference signal is located at a frequency lower than that of the electrical low-speed channels. 
   
   
     6. The method of  claim 1  wherein:
 each of the transmitter node and the receiver node includes a local oscillator; 
 synchronizing the transmitter node with the reference signal comprises locking a local oscillator used in the transmitter node to the reference signal; and 
 synchronizing the receiver node with the recovered reference signal comprises locking a local oscillator used in the receiver node to the recovered reference signal. 
 
   
   
     7. The method of  claim 6  wherein:
 modulating the reference signal onto an optical signal comprises:
 generating a harmonic of the reference signal; and 
 modulating the harmonic onto the optical signal; and 
 
 recovering the reference signal from the optical signal comprises:
 recovering the harmonic from the optical signal; and 
 frequency dividing the harmonic to recover the reference signal. 
 
 
   
   
     8. The method of  claim 7  wherein:
 the substep of modulating the harmonic onto an optical signal comprises:
 frequency division multiplexing the harmonic with a plurality of electrical low-speed channels to form an electrical high-speed channel; and 
 converting the electrical high-speed channel from electrical to optical form to form the optical signal; and 
 
 the substep of recovering the harmonic from the optical signal comprises:
 converting the optical signal from optical to electrical form to recover the electrical high-speed channel; and 
 frequency division demultiplexing the harmonic from the electrical high-speed channel. 
 
 
   
   
     9. The method of  claim 8  wherein, in the electrical high-speed channel, the harmonic is located at a frequency lower than that of the electrical low-speed channels. 
   
   
     10. The method of  claim 8  wherein each of the electrical low-speed channels and the harmonic is allocated a different frequency band within the electrical high-speed channel and within the optical signal. 
   
   
     11. An optical fiber communications system for transmitting at least two low-speed channels across the communications system, the communications system comprising:
 a transmitter node including:
 a local oscillator for generating a reference signal; and 
 an FDM multiplexer coupled to the local oscillator for combining the low-speed channels with the reference signal into an electrical high-speed channel, wherein the reference signal and the low speed channels are separate signals in the electrical high-speed channel, wherein each of the low-speed channels and the reference signal is allocated a different frequency band within the electrical high-speed channel; and 
 
 a receiver node coupled to the transmitter node by an optical fiber, the receiver node including:
 an FDM demultiplexer for recovering the reference signal from the electrical high-speed channel; 
 a local oscillator; and 
 electronics coupled to the local oscillator and the FDM demultiplexer for synchronizing the local oscillator with the recovered reference signal. 
 
 
   
   
     12. The optical fiber communications system of  claim 11  wherein:
 the transmitter node further includes:
 electronics coupled between the local oscillator and the FDM multiplexer for generating a pilot tone from a harmonic of the reference signal, wherein the FDM multiplexer combines the low-speed channels with the pilot tone into an electrical high-speed channel; and 
 an E/O converter coupled to the FDM multiplexer for converting the electrical high-speed channel into an optical high-speed channel; and 
 
 the receiver node further includes:
 an O/E converter coupled to the FDM demultiplexer for receiving the optical high-speed channel and converting it to the electrical high-speed channel; 
 wherein the FDM demultiplexer recovers the pilot tone from the electrical high-speed channel and the electronics in the receiver node recover the reference signal from the pilot tone. 
 
 
   
   
     13. The optical fiber communications system of  claim 12  wherein, in the electrical high-speed channel, the pilot tone is located at a frequency lower than that of the electrical low-speed channels. 
   
   
     14. The optical fiber communications system of  claim 12  wherein each of the electrical low-speed channels and the pilot tone is allocated a different frequency band within the electrical high-speed channel and within the optical high-speed channel. 
   
   
     15. A transmitter node for transmitting at least two low-speed channels across an optical fiber communications system, wherein the transmitter node comprises:
 a generator configured to generate a reference signal; 
 a synchronizer configured to adjust a data rate of a data signal to match a rate of the reference signal; 
 a modulator configured to modulate the reference signal onto an optical signal in a first frequency band and modulate the data signal onto the optical signal in a second frequency band, the first frequency band being different from the second frequency band; and 
 circuitry configured to transmit the optical signal across the optical fiber. 
 
   
   
     16. The transmitter node of  claim 15 ,
 wherein the generator is further configured to generate a harmonic of the reference signal; and 
 wherein to modulate the reference signal onto the optical signal, the modulator is further configured to modulate the harmonic onto the optical signal. 
 
   
   
     17. The transmitter node of  claim 15 ,
 wherein the modulator includes an FDM multiplexer; 
 wherein to modulate the reference signal and the data signal onto an optical signal, the FDM multiplexer is configured to frequency division multiplexing the reference signal with a plurality of electrical low-speed channels to form an electrical high-speed channel; and 
 wherein an electrical to optical (L/O) converter is configured to convert the electrical high-speed channel from electrical to optical form to form the optical signal. 
 
   
   
     18. The transmitter node of  claim 17  wherein, within the electrical high-speed channel, the reference signal is located at a frequency lower than that of the electrical low-speed channels. 
   
   
     19. The transmitter node of  claim 15 ,
 wherein the transmitter node includes a local oscillator; and 
 wherein the transmitter node is further configured to lock the local oscillator to the reference signal. 
 
   
   
     20. The transmitter node of  claim 19 ,
 wherein the transmitter node is further configured to generate a pilot tone from a harmonic of the reference signal; and 
 wherein to modulate the reference signal onto the optical signal, the modulator is further configured to modulate the pilot tone onto the optical signal. 
 
   
   
     21. The transmitter node of  claim 20 , wherein to modulate the pilot tone onto an optical signal, the FDM multiplexer is configured to frequency division multiplex the pilot tone with a plurality of electrical low-speed channels to form an electrical high-speed channel. 
   
   
     22. The transmitter node of  claim 21  wherein, within the electrical high-speed channel, the pilot tone is located at a frequency lower than that of the electrical low-speed channels. 
   
   
     23. The transmitter node of  claim 21  wherein each of the electrical low-speed channels and the pilot tone is allocated a different frequency band within the electrical high-speed channel and within the optical signal. 
   
   
     24. A receiver node for receiving at least two low-speed channels transmitted across an optical fiber communications system, wherein the receiver node comprises:
 circuitry configured to receive an optical signal; 
 a demodulator configured to recover a reference signal and a data signal from the optical signal, wherein the reference signal is modulated onto the optical signal in a first frequency band and the data signal is modulated onto the optical signal in a second frequency band, the first frequency band being different from the second frequency band; and 
 a synchronizer configured to adjust a data rate of the recovered data signal to match a rate of the recovered reference signal. 
 
   
   
     25. The receiver node of  claim 24 , wherein to recover the reference signal from the optical signal, the demodulator is further configured to:
 recover a harmonic of the reference signal from the optical signal; and 
 frequency divide the harmonic to recover the reference signal. 
 
   
   
     26. The receiver node of  claim 24 ,
 wherein the receiver node includes an electrical to optical (O/L) converter coupled to an FDM demultiplexer; 
 wherein the O/E converter is configured to convert the optical signal from optical to electrical form to recover the electrical high-speed channel; and 
 wherein the FDM demultiplexer is configured to frequency division demultiplexing the reference signal from the electrical high-speed channel. 
 
   
   
     27. The receiver node of  claim 26  wherein, in the electrical high-speed channel, the reference signal is located at a frequency lower than that of the electrical low-speed channels. 
   
   
     28. The receiver node of  claim 24 , wherein the receiver node is further configured to lock the second local oscillator to the recovered reference signal. 
   
   
     29. The receiver node of  claim 28 , wherein to recover the reference signal from the optical signal, the demodulator is further configured to:
 recover a pilot tone from the optical signal; and 
 frequency divide the pilot tone to recover the reference signal. 
 
   
   
     30. The receiver node of  claim 29 , wherein to recover the pilot tone from the optical signal, the FDM demultiplexer is configured to frequency division demultiplex the pilot tone from the electrical high-speed channel. 
   
   
     31. The receiver node of  claim 30  wherein, in the electrical high-speed channel, the pilot tone is located at a frequency lower than that of the electrical low-speed channels. 
   
   
     32. The receiver node of  claim 30  wherein each of the electrical low-speed channels and the pilot tone is allocated a different frequency band within the electrical high-speed channel and within the optical high-speed channel. 
   
   
     33. An optical fiber communications system for transmitting at least two low-speed channels across the communications system, the communications system comprising:
 a transmitter node configured to:
 generate a reference signal; 
 modulate the reference signal onto an optical signal in a first frequency band and modulate the data signal onto the optical signal in a second frequency band, the first frequency band being different from the second frequency band; and 
 transmit the optical signal across the optical fiber; and 
 
 a receiver node configured to:
 receive the optical signal; 
 recover the reference signal and the data signal from the optical signal; and 
 adjust a data rate of the recovered data signal to match a rate of the recovered reference signal. 
 
 
   
   
     34. The optical fiber communications system of  claim 33 , wherein the transmitter node is further configured to adjust the data rate of a data signal to match the rate of the reference signal. 
   
   
     35. The optical fiber communications system of  claim 33 , wherein to modulate the reference signal onto an optical signal, the transmitter node is further configured to:
 generate a harmonic of the reference signal; and 
 modulate the harmonic onto the optical signal. 
 
   
   
     36. The optical fiber communications system of  claim 35 , wherein the receiver node is further configured to:
 recover the harmonic from the optical signal; and 
 frequency divide the harmonic to recover the reference signal. 
 
   
   
     37. The optical fiber communications system of  claim 33 ,
 wherein the transmitter node includes an FDM multiplexer coupled to an electrical to optical (L/O) converter; 
 wherein to modulate the reference signal and the data signal onto an optical signal, the FDM multiplexer is configured to frequency division multiplexing the reference signal with a plurality of electrical low-speed channels to form an electrical high-speed channel; and 
 wherein the E/O converter is configured to convert the electrical high-speed channel from electrical to optical form to form the optical signal. 
 
   
   
     38. The optical fiber communications system of  claim 37 ,
 wherein the receiver node includes an O/E converter coupled to an FDM demultiplexer; 
 wherein the O/E converter is configured to convert the optical signal from optical to electrical form to recover the electrical high-speed channel; and 
 wherein the FDM demultiplexer is configured to frequency division demultiplexing the reference signal from the electrical high-speed channel. 
 
   
   
     39. The optical fiber communications system of  claim 38  wherein, in the electrical high-speed channel, the reference signal is located at a frequency lower than that of the electrical low-speed channels. 
   
   
     40. The optical fiber communications system of  claim 33 ,
 wherein the transmitter node includes a first local oscillator; and 
 wherein the transmitter node is further configured to lock the first local oscillator to the reference signal. 
 
   
   
     41. The optical fiber communications system of  claim 40 , wherein to modulate the reference signal onto an optical signal, the transmitter node is further configured to:
 generate a pilot tone from a harmonic of the reference signal; and 
 modulate the pilot tone onto the optical signal. 
 
   
   
     42. The optical fiber communications system of  claim 41  wherein to modulate the pilot tone onto an optical signal, the FDM multiplexer is configured to frequency division multiplex the pilot tone with a plurality of electrical low-speed channels to form an electrical high-speed channel. 
   
   
     43. The optical fiber communications system of  claim 41 , wherein the receiver node is further configured to:
 recover the pilot tone from the optical signal; and 
 frequency divide the pilot tone to recover the reference signal. 
 
   
   
     44. The optical fiber communications system of  claim 33 ,
 wherein the receiver node includes a second local oscillator; and 
 wherein the receiver node is further configured to lock the second local oscillator to the recovered reference signal. 
 
   
   
     45. The optical fiber communications system of  claim 44 , wherein to recover the pilot tone from the optical signal, the FDM demultiplexer is configured to frequency division demultiplex the pilot tone from the electrical high-speed channel. 
   
   
     46. In an optical fiber communications system including a transmitter node coupled to a receiver node by an optical fiber, a method for synchronizing the receiver node with the transmitter node, the method comprising:
 at the transmitter node:
 generating a reference signal; 
 modulating the reference signal onto an optical signal in a first frequency band and modulating the data signal onto the optical signal in a second frequency band, the first frequency band being different from the second frequency band; and 
 transmitting the optical signal across the optical fiber; and 
 
 at the receiver node:
 receiving the optical signal; 
 recovering the reference signal and the data signal from the optical signal; and 
 adjusting a data rate of the recovered data signal to match a rate of the recovered reference signal. 
 
 
   
   
     47. The method of  claim 46 , further comprising adjusting a data rate of a data signal at the transmitter node to match a rate of the reference signal. 
   
   
     48. The method of  claim 46 , wherein modulating the reference signal and the data signal onto an optical signal comprises:
 generating a harmonic of the reference signal; and 
 modulating the harmonic onto the optical signal. 
 
   
   
     49. The method of  claim 48 , further comprising:
 recovering the harmonic from the optical signal at the receiver node; and 
 frequency dividing the harmonic to recover the reference signal. 
 
   
   
     50. The method of  claim 46 , further comprising:
 frequency division multiplexing the reference signal with a plurality of electrical low-speed channels to form an electrical high-speed channel at the transmitter node; and 
 converting the electrical high-speed channel from electrical to optical form to form the optical signal. 
 
   
   
     51. The method of  claim 50 , further comprising:
 converting the optical signal from optical to electrical form at the receiver node to recover the electrical high-speed channel; and
 frequency division demultiplexing the reference signal from the electrical high-speed channel. 
 
 
   
   
     52. The method of  claim 51  wherein in the electrical high-speed channel, the reference signal is located at a frequency lower than that of the electrical low-speed channels. 
   
   
     53. The method of  claim 46 , wherein the transmitter node includes a first local oscillator, the method further comprising locking the first local oscillator to the reference signal. 
   
   
     54. The method of  claim 53 , wherein modulating the reference signal and the data signal onto an optical signal comprises:
 generating a pilot tone from a harmonic of the reference signal at the transmitter node; and 
 modulating the pilot tone onto the optical signal. 
 
   
   
     55. The method of  claim 54  wherein modulating the pilot tone onto an optical signal further comprises frequency division multiplexing the pilot tone with a plurality of electrical low-speed channels to form an electrical high-speed channel. 
   
   
     56. The method of  claim 54 , further comprising:
 recovering the pilot tone from the optical signal; and 
 frequency dividing the pilot tone to recover the reference signal. 
 
   
   
     57. The method of  claim 56 , wherein the receiver node includes a second local oscillator, the method further comprising locking the second local oscillator to the recovered reference signal. 
   
   
     58. The method of  claim 56 , wherein recovering the pilot tone from the optical signal further comprises frequency division demultiplexing the pilot tone from the electrical high-speed channel. 
   
   
     59. A method for transmitting at least two low-speed channels across an optical fiber communications system, the method comprising:
 generating a reference signal; 
 adjusting a data rate of a data signal to match a rate of the reference signal; 
 modulating the reference signal onto an optical signal in a first frequency band and modulating the data signal onto the optical signal in a second frequency band, the first frequency band being different from the second frequency band; and 
 transmitting the optical signal across the optical fiber. 
 
   
   
     60. The method of  claim 59 , further comprising:
 generating a harmonic of the reference signal; and 
 modulating the harmonic onto the optical signal. 
 
   
   
     61. The method of  claim 59 , further comprising:
 frequency division multiplexing the reference signal with a plurality of electrical low-speed channels to form an electrical high-speed channel; and 
 converting the electrical high-speed channel from electrical to optical form to form the optical signal. 
 
   
   
     62. The method of  claim 61  wherein, within the electrical high-speed channel, the reference signal is located at a frequency lower than that of the electrical low-speed channels. 
   
   
     63. The method of  claim 59 , further comprising locking a local oscillator to the reference signal. 
   
   
     64. The method of  claim 63 , further comprising:
 generating a pilot tone from a harmonic of the reference signal; and 
 modulating the pilot tone onto the optical signal. 
 
   
   
     65. The method of  claim 64 , wherein to modulate the pilot tone onto an optical signal, the method comprises frequency division multiplexing the pilot tone with a plurality of electrical low-speed channels to form an electrical high-speed channel. 
   
   
     66. The method of  claim 65  wherein, within the electrical high-speed channel, the pilot tone is located at a frequency lower than that of the electrical low-speed channels. 
   
   
     67. A method for receiving at least two low-speed channels transmitted across an optical fiber communications system, the method comprising:
 receiving the optical signal; 
 recovering a reference signal and a data signal from the optical signal, wherein the reference signal is modulated onto the optical signal in a first frequency band and the data signal is modulated onto the optical signal in a second frequency band, the first frequency band being different from the second frequency band; and 
 adjusting a data rate of the recovered data signal to match a rate of the recovered reference signal. 
 
   
   
     68. The method of  claim 67 , wherein recovering the reference signal from the optical signal further comprises:
 recovering a harmonic of the reference signal from the optical signal; and 
 frequency dividing the harmonic. 
 
   
   
     69. The method of  claim 67 , further comprising:
 converting the optical signal from optical to electrical form to recover the electrical high-speed channel; and 
 frequency division demultiplexing the reference signal from the electrical high-speed channel. 
 
   
   
     70. The method of  claim 69  wherein, in the electrical high-speed channel, the reference signal is located at a frequency lower than that of the electrical low-speed channels. 
   
   
     71. The method of  claim 67 , further comprising locking a local oscillator to the recovered reference signal. 
   
   
     72. The method of  claim 71 , wherein to recover the reference signal, the method further comprises:
 recovering a pilot tone from the optical signal; and 
 frequency dividing the pilot tone. 
 
   
   
     73. The method of  claim 72 , wherein to recover the pilot tone from the optical signal, the method comprises frequency division demultiplexing the pilot tone from the electrical high-speed channel. 
   
   
     74. The method of  claim 73  wherein, in the electrical high-speed channel, the pilot tone is located at a frequency lower than that of the electrical low-speed channels. 
   
   
     75. The method of  claim 73  wherein each of the electrical low-speed channels and the pilot tone is allocated a different frequency band within the electrical high-speed channel and within the optical high-speed channel.